In many nuclear fuel bundles for boiling water reactors, the fuel bundle contains an array of full-length fuel rods and a considerably lesser number of part-length fuel rods. The part-length fuel rods define steam vent volumes within the bundle and channel in the spaces above the part-length rods. It will be appreciated that the steam vent volumes do not themselves produce power. The part-length fuel rods and the associated steam vent volumes are a trade-off between critical power production and the desirable pressure drop reduction capability of the steam vent volumes. It will also be appreciated that the overall objective of any nuclear fuel bundle design is to optimize the performance of each fuel rod. The overall performance of the bundle, however, is no better than the performance of any individual fuel rod and, consequently, critical power is a function of the power achieved by the bundle before the first rod overheats.
Recently, it has been demonstrated that the region above the individual part-length fuel rods can be equipped with critical power enhancement devices such as swirlers. Those devices improve the critical power performance of the surrounding rods without negating the pressure drop reduction capability of the steam vent volume. However, the location of the steam vent volumes within the fuel bundle is believed not to have been considered previously with respect to its impact on the critical power performance of the fuel rods.